[PATCH 33/38] fs: fat: Support reading from a larger block size
Simon Glass
sjg at chromium.org
Thu Mar 30 23:32:22 CEST 2023
At present it is not possible to read from some CDROM drives since the
FAT sector size does not match the media's block size. Add a conversion
option for this, so that reading is possible.
This does increase SPL size for read-only FAT support by 25 bytes but
all but 6 are covered by the previous patch. We could reduce the
overhead of this feature to 0 bytes by making the code uglier (using
a static variable).
Signed-off-by: Simon Glass <sjg at chromium.org>
---
fs/fat/Kconfig | 13 ++++++
fs/fat/fat.c | 107 ++++++++++++++++++++++++++++++++++++++++-----
fs/fat/fat_write.c | 8 ++--
3 files changed, 114 insertions(+), 14 deletions(-)
diff --git a/fs/fat/Kconfig b/fs/fat/Kconfig
index 9bb11eac9f7a..b0aa888c6cc4 100644
--- a/fs/fat/Kconfig
+++ b/fs/fat/Kconfig
@@ -22,3 +22,16 @@ config FS_FAT_MAX_CLUSTSIZE
is the smallest amount of disk space that can be used to hold a
file. Unless you have an extremely tight memory memory constraints,
leave the default.
+
+config FAT_BLK_XLATE
+ bool "Enable FAT filesystem on a device with a larger block size"
+ depends on FS_FAT
+ help
+ This provides a simple translation mechanism for reading FAT
+ filesystems which don't use the same sector size as the underlying
+ media. For example, the FAT filesystem may use 512 bytes but the
+ media uses 2048, e.g. on a CDROM drive.
+
+ This only supports the case where the FAT filesystem's sector size is
+ smaller than the media's block size. It does not support creating or
+ writing files.
diff --git a/fs/fat/fat.c b/fs/fat/fat.c
index f0df7988e172..02c6d55a99b3 100644
--- a/fs/fat/fat.c
+++ b/fs/fat/fat.c
@@ -43,13 +43,93 @@ static struct disk_partition cur_part_info;
#define DOS_FS_TYPE_OFFSET 0x36
#define DOS_FS32_TYPE_OFFSET 0x52
-static int disk_read(__u32 block, __u32 nr_blocks, void *buf)
+/**
+ * disk_read_conv() - Read blocks and break them into smaller ones
+ *
+ * This is used when the FAT filesystem is hosted on a block device with a
+ * block size greated than 512 bytes, e.g. the 2048 bytes of a CDROM drive. It
+ * reads the blocks into a buffer and pulls out what is requested by the calling
+ * function.
+ *
+ * It uses an internal 2KB buffer on the stack.
+ *
+ * @mydata: Filesystem information
+ * @block: Block number to read, in terms of mydata->sect_size
+ * @nr_blocks: Number of blocks to read, in terms of mydata->sect_size
+ * @buf: Buffer for data
+ */
+static int disk_read_conv(fsdata *mydata, __u32 block, __u32 nr_blocks,
+ void *buf)
+{
+ uint factor, whole, remain, upto;
+ ulong base, index;
+ uint to_copy;
+ u8 tbuf[2048];
+ int ret;
+
+ log_debug("mydata %x, cur_dev %lx, block %x, nr_block %x\n",
+ mydata->sect_size, cur_dev->blksz, block, nr_blocks);
+ if (mydata->sect_size > cur_dev->blksz ||
+ cur_dev->blksz > sizeof(tbuf)) {
+ log_err("Block size %lx not supported\n", cur_dev->blksz);
+ return -EIO;
+ }
+ factor = cur_dev->blksz / mydata->sect_size;
+
+ /* get the first partial block */
+ base = cur_part_info.start + block / factor;
+ index = block % factor;
+ log_debug("cur_part_info.start %llx, block %x, base %lx, index %lx\n",
+ (unsigned long long)cur_part_info.start, block, base, index);
+ ret = blk_dread(cur_dev, base, 1, tbuf);
+ if (ret != 1)
+ return -EIO;
+
+ to_copy = min((ulong)nr_blocks, factor - index);
+ log_debug("to_copy %x\n", to_copy);
+ memcpy(buf, tbuf + index * mydata->sect_size,
+ to_copy * mydata->sect_size);
+ upto = to_copy;
+
+ /* load any whole blocks */
+ remain = nr_blocks - upto;
+ whole = remain / factor;
+ log_debug("factor %x, whole %x, remain %x\n", factor, whole, remain);
+ if (whole) {
+ ret = blk_dread(cur_dev, base + 1, whole,
+ buf + upto * mydata->sect_size);
+ if (ret != whole)
+ return -EIO;
+ upto += whole * factor;
+ remain = nr_blocks - upto;
+ }
+
+ /* load any blocks at the end */
+ log_debug("end: remain %x\n", remain);
+ if (remain) {
+ ret = blk_dread(cur_dev, base + 1 + whole, 1, tbuf);
+ if (ret != 1)
+ return -EIO;
+ memcpy(buf + upto * mydata->sect_size, tbuf,
+ remain * mydata->sect_size);
+ upto += remain;
+ }
+
+ return upto;
+}
+
+static int disk_read(fsdata *mydata, __u32 block, __u32 nr_blocks, void *buf)
{
ulong ret;
if (!cur_dev)
return -1;
+ /* support converting from a larger block size */
+ if (IS_ENABLED(CONFIG_FAT_BLK_XLATE) && mydata &&
+ mydata->sect_size != cur_dev->blksz)
+ return disk_read_conv(mydata, block, nr_blocks, buf);
+
ret = blk_dread(cur_dev, cur_part_info.start + block, nr_blocks, buf);
if (ret != nr_blocks)
@@ -66,7 +146,7 @@ int fat_set_blk_dev(struct blk_desc *dev_desc, struct disk_partition *info)
cur_part_info = *info;
/* Make sure it has a valid FAT header */
- if (disk_read(0, 1, buffer) != 1) {
+ if (disk_read(NULL, 0, 1, buffer) != 1) {
cur_dev = NULL;
return -1;
}
@@ -211,7 +291,7 @@ static __u32 get_fatent(fsdata *mydata, __u32 entry)
if (flush_dirty_fat_buffer(mydata) < 0)
return -1;
- if (disk_read(startblock, getsize, bufptr) < 0) {
+ if (disk_read(mydata, startblock, getsize, bufptr) < 0) {
debug("Error reading FAT blocks\n");
return ret;
}
@@ -265,7 +345,7 @@ get_cluster(fsdata *mydata, __u32 clustnum, __u8 *buffer, unsigned long size)
debug("FAT: Misaligned buffer address (%p)\n", buffer);
while (size >= mydata->sect_size) {
- ret = disk_read(startsect++, 1, tmpbuf);
+ ret = disk_read(mydata, startsect++, 1, tmpbuf);
if (ret != 1) {
debug("Error reading data (got %d)\n", ret);
return -1;
@@ -279,7 +359,7 @@ get_cluster(fsdata *mydata, __u32 clustnum, __u8 *buffer, unsigned long size)
__u32 bytes_read;
__u32 sect_count = size / mydata->sect_size;
- ret = disk_read(startsect, sect_count, buffer);
+ ret = disk_read(mydata, startsect, sect_count, buffer);
if (ret != sect_count) {
debug("Error reading data (got %d)\n", ret);
return -1;
@@ -292,7 +372,7 @@ get_cluster(fsdata *mydata, __u32 clustnum, __u8 *buffer, unsigned long size)
if (size) {
ALLOC_CACHE_ALIGN_BUFFER(__u8, tmpbuf, mydata->sect_size);
- ret = disk_read(startsect, 1, tmpbuf);
+ ret = disk_read(mydata, startsect, 1, tmpbuf);
if (ret != 1) {
debug("Error reading data (got %d)\n", ret);
return -1;
@@ -504,7 +584,7 @@ read_bootsectandvi(boot_sector *bs, volume_info *volinfo, int *fatsize)
return -1;
}
- if (disk_read(0, 1, block) < 0) {
+ if (disk_read(NULL, 0, 1, block) < 0) {
debug("Error: reading block\n");
goto fail;
}
@@ -586,9 +666,14 @@ static int get_fs_info(fsdata *mydata)
mydata->sect_size = (bs.sector_size[1] << 8) + bs.sector_size[0];
mydata->clust_size = bs.cluster_size;
if (mydata->sect_size != cur_part_info.blksz) {
- printf("** FAT sector size mismatch (fs=%hu, dev=%lu)\n",
- mydata->sect_size, cur_part_info.blksz);
- return -1;
+ if (IS_ENABLED(CONFIG_FAT_BLK_XLATE)) {
+ printf("Warning: FAT sector size mismatch (fs=%u, dev=%lu): translating for read-only\n",
+ mydata->sect_size, cur_part_info.blksz);
+ } else {
+ printf("** FAT sector size mismatch (fs=%u, dev=%lu), see CONFIG_FAT_BLK_XLATE\n",
+ mydata->sect_size, cur_part_info.blksz);
+ return -1;
+ }
}
if (mydata->clust_size == 0) {
printf("** FAT cluster size not set\n");
@@ -846,7 +931,7 @@ void *fat_next_cluster(fat_itr *itr, unsigned int *nbytes)
* dent at a time and iteratively constructing the vfat long
* name.
*/
- ret = disk_read(sect, read_size, itr->block);
+ ret = disk_read(itr->fsdata, sect, read_size, itr->block);
if (ret < 0) {
debug("Error: reading block\n");
return NULL;
diff --git a/fs/fat/fat_write.c b/fs/fat/fat_write.c
index 4d2d4db07fa6..baef52af6363 100644
--- a/fs/fat/fat_write.c
+++ b/fs/fat/fat_write.c
@@ -474,7 +474,7 @@ static int set_fatent_value(fsdata *mydata, __u32 entry, __u32 entry_value)
startblock += mydata->fat_sect;
- if (disk_read(startblock, getsize, bufptr) < 0) {
+ if (disk_read(NULL, startblock, getsize, bufptr) < 0) {
debug("Error reading FAT blocks\n");
return -1;
}
@@ -709,7 +709,8 @@ get_set_cluster(fsdata *mydata, __u32 clustnum, loff_t pos, __u8 *buffer,
/* partial write at beginning */
if (pos) {
wsize = min(bytesperclust - pos, size);
- ret = disk_read(startsect, mydata->clust_size, tmpbuf_cluster);
+ ret = disk_read(NULL, startsect, mydata->clust_size,
+ tmpbuf_cluster);
if (ret != mydata->clust_size) {
debug("Error reading data (got %d)\n", ret);
return -1;
@@ -775,7 +776,8 @@ get_set_cluster(fsdata *mydata, __u32 clustnum, loff_t pos, __u8 *buffer,
/* partial write at end */
if (size) {
wsize = size;
- ret = disk_read(startsect, mydata->clust_size, tmpbuf_cluster);
+ ret = disk_read(NULL, startsect, mydata->clust_size,
+ tmpbuf_cluster);
if (ret != mydata->clust_size) {
debug("Error reading data (got %d)\n", ret);
return -1;
--
2.40.0.348.gf938b09366-goog
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